The present invention relates to a method and a control device for setting an actual value of a turbine flow cross section of a motor vehicle internal combustion engine turbocharger for a change in the load on the internal combustion engine from a larger load value to a smaller load value. The time derivative of the load is then negative, so that such a load change may also be referred to as negative load change.
“Load on the internal combustion engine” is primarily understood to be the actual fuel charge normalized to a maximum possible fuel charge of a combustion chamber of the internal combustion engine with a combustible mixture. The relative fuel charge is an essential manipulated variable for setting the torque supplied by the internal combustion engine.
The aforementioned method is known from the “Die Bibliothek der Technik [Technology Library] series, Volume 103, Exhaust Gas Turbochargers,” Verlag Moderne Industrie, D-86896 Landsberg/Lech, ISBN 3-478-93263-7, page 40. This literature reference deals with a turbocharger having variable turbine geometry (VTG) in which the turbine flow cross section is reduced by closing guide blades in order to produce a greater pressure drop between the turbine inlet and the turbine outlet. For an acceleration from low rotational speeds the guide blades should be closed to obtain the maximum energy from the exhaust gas. The blades also open with increasing rotational speed and adapt to the particular operating point.
The guide blades are situated in an annular gap through which exhaust gas flows from the outside. A radial orientation of the guide blades results in a smaller turbine flow cross section than for a tangential orientation of the guide blades. For spark ignition engines, the exhaust gas temperature upstream from the turbine may fluctuate in a range from several hundred ° C. to greater than 1000° C. Due to the resulting high thermal load on the guide blades and the soot and/or carbonized oil particles that accumulate over time upstream from the turbine, the guide blades in the annular cross section may jam.
In known systems, jamming of the guide blades is detected by onboard diagnostics during operation of the motor vehicle, and the internal combustion engine is switched to emergency mode to avoid consequent damage to the turbocharger and/or the internal combustion engine. This is noticeable to the driver and is perceived as a drawback. When the guide blades jam in an almost open position, this results in, for example, significant loss of the maximum achievable torque when accelerating from initially low rotational speeds.
In light of this background, an object of the present invention is to provide a method and a control device by which switching to emergency mode is avoided to the greatest extent possible, without having to deal with the described consequent damages and/or losses of the maximum achievable torque.
This object has been achieved by a method in which an actual value of the turbine flow cross section to be set for the smaller load value is set in a delayed manner to a setpoint value that is specified for the smaller load value under stationary conditions.
Correspondingly, this object has been further achieved by a device that controls the progression of such a method.
The present invention is based on the finding that jamming usually occurs after a negative load change in the almost open state.
As the result of setting the turbine flow cross section to the setpoint value that is valid for stationary conditions in a delayed manner according to the present invention after a negative load change, to a certain extent the previous history of the instantaneous operating point is taken into account. With regard to the tendency of guide blades to jam, it is thus possible to avoid the critical combination of still hot guide blades and the open guide blade position. As a result, the frequency of jamming of the guide blades and/or their adjustment mechanisms may be significantly reduced.
It is preferable for the setpoint value that is to be specified under stationary conditions to be set by specifying a progression of setpoint values that results in the setpoint value that is to be specified under stationary conditions.
This ensures that the setpoint value that is valid for stationary conditions is achieved when the tendency to jam has been reduced. Specifying a progression of setpoint values allows the reduction of the jamming tendency to be optimized.
It is also preferable for the progression to be specified as a function of operating parameters of the internal combustion engine.
Thus, at least one of the following variables is typically considered: combustion chamber fuel charge, rotational speed, exhaust gas temperature, and cumulative mass air flow rate.
Each of these variables, alone or in combination with one or more of the other variables, represents a measure of the heat transported by the exhaust gas mass flow, and thus also for the heating and risk of jamming. This embodiment thus allows the adjustment of the guide blades to be limited or delayed as a function of a modeled jamming risk.
It is also preferable for the operating parameters to be dependent on the surroundings in which the internal combustion engine is operated. Such operating parameters preferably include at least one operating parameter which indicates a pressure or a temperature in the intake system of the internal combustion engine.
This embodiment is based on the finding that, for example, operation at high altitudes with low ambient pressure allows an intermediate position of the blades to be maintained for a longer period, because the associated increased air demand of the turbocharger due to the low ambient pressure does not yet result in critically high combustion chamber fuel charge. Therefore, under these conditions the referenced progression can be extended in time.
Low charge air temperatures are accompanied by correspondingly reduced exhaust gas temperatures, and because of the associated cooling effect allow the progression to be compressed in time.
It is also preferable for the progression to have at least one intermediate value of the turbine flow cross section that is larger than the turbine flow cross section set for the larger load value, and is smaller than the turbine flow cross section that is to be set for the smaller load value under stationary conditions. In other words, the increase in the turbine flow cross section is limited after a negative load change. The guide blades are operated in a position that is further open, but not to the extent as for operation under stationary conditions.
It has been shown that jamming usually occurs in the almost open state, and is facilitated by high temperatures. Thus, jamming occurs in particular in a cumulative manner following a closed position, which is the setting during acceleration. Acceleration with a subsequent extreme reduction in the torque demand thus represents an example of such a critical combination of the instantaneous operating point and its previous history. This critical combination is avoided by the described embodiment.
It is also preferable for a time duration of the delay to be limited to a predetermined maximum value.
As a result of this embodiment, the influence of the previous history is no longer taken into account after achieving the maximum value of the time duration. In this manner, temperature normalization may be taken into consideration which, for example, occurs some time after an acceleration phase as the result of exhaust gas which is once again cooler.
It is also contemplated for a check to be made as to whether a guide blade adjustment mechanism is jammed, and for a breakaway function to be activated in the event of a jam. This embodiment is employed when the guide blades have jammed despite the above-referenced measures. In many cases, this breakaway function causes the previous jammed guide blades to become unjammed, so that switching to emergency mode may be avoided. In another embodiment the breakaway function is periodically repeated. In this way, even severe jamming may be remedied over time and in conjunction with variable exhaust gas temperatures.
In a still further embodiment, a frequency of occurrence of a jamming guide blade adjustment mechanism is determined, and an emergency program is activated when the frequency exceeds a predetermined threshold value.
Jamming guide blades alter the operating characteristics of the internal combustion engine, and result in, for example, changes in the charging pressure and the exhaust gas back pressure. The tolerance for jamming conditions drops with the frequency of their occurrence. Taking into account the frequency when switching to emergency operation avoids switching when the frequency is still tolerable, without preventing switching when the frequency is no longer tolerable. The same advantages are obtained for corresponding embodiments of the control device.